1. The Field of the Invention
The invention relates generally to oscillators, and more specifically, to an oscillator that has a rapid and predictable startup time by stimulating the oscillator terminals with a controlled Alternating Current (AC) voltage source.
2. Background and Related Art
Wireless communication has revolutionized our way of life by allowing devices and their associated users to communicate without the device having to be connected to a fixed communication interface. Instead, wireless devices are capable of communicating over wireless networks even while mobile. As such devices are mobile, they often are not connected to a fixed power source, and must instead rely on battery power. In order to reduce the user""s burden in having to recharge or replace the battery, it is often advantageous for such devices to preserve battery power by having a low power mode in which all but the most critical circuits run on low power or are turned off entirely when the user is not actively using the device. The device then at least partially powers up again once the need arises.
The time that it takes for a device to transition from the lower power mode ito the increased power mode needed for proper operation is often referred to as the xe2x80x9cstartup timexe2x80x9d of the device. A slower startup time can significantly degrade the performance of the device depending on the application. In some cases, the device should start up in a matter of microseconds in order to achieve acceptable performance. In many devices, the limiting circuit element that tends to provide a lower limit on startup time is the crystal oscillator. The crystal oscillator is a component of many circuits that rely on a signal having a given and known frequency. While wireless communication devices have been previously mentioned, the crystal oscillator may be used for purposes that are not communication-oriented such as for providing a clock signal of a given and known frequency. The startup time of the crystal oscillator is also unpredictable and can vary even within the same device. In many conventional circuits, the crystal oscillator is allowed time to start up by amplifying its own internally generated noise and repeatedly amplifying the resulting signal using a feedback loop.
FIG. 4 illustrates a conventional crystal oscillator circuit 400 in accordance with the prior art. The circuit 400 includes a resonating element 401 that provides the noise for amplification. The resonating element 401 may be any circuit capable of resonating at a particular frequency or a range of frequencies. Accordingly, the resonating element 401 may be a ceramic resonator, any inductor-capacitor equivalent circuit, but most typically is a crystal. The crystal is conventionally used due to its reliability in providing a relatively stable resonating frequency over long periods of time.
One terminal of the resonating element 401 is provided as input to an inverting aO amplifier 402, which outputs the amplified signal to the other terminal of the resonating element 401. Capacitor 404 (also identified as element C1) has one terminal connected to a terminal of the resonating element 401 and the other terminal connected to ground.
Likewise, capacitor 405 (also identified as element C2) has one terminal connected to the other terminal of the resonating element 401 and the other terminal connected to ground. The capacitors thus configured allow the terminals of the resonating element 401 to support an Alternating Current (AC) signal. Resistor 403 (also identified as element RF) is used to bias up the amplifier and complete the Direct Current (DC) feedback loop while providing some AC gain isolation.
As time proceeds forward, the differential AC voltage applied at the terminals of the resonating element 401 will become of sufficient magnitude to be useful. In the case of FIG. 4, the differential AC voltage may be applied as input to the buffer 406, which uses the differential AC voltage to generate a clock signal of the same primary frequency. In that case, the startup time is the time required for the crystal oscillator to amplify the internally generated noise to a differential AC voltage of sufficient magnitude that the buffer 406 may generate a useful clock signal.
This conventional crystal oscillator circuit 400 is useful for many applications that do not require rapid startup times. Typical startup times for such a crystal oscillator circuit can be as high as from one to ten milliseconds. Furthermore, the circuit requires relatively few devices and thus is relatively inexpensive to manufacture. However, sometimes, such long startup times may be unacceptable. Accordingly, there have been conventional attempts to reduce the startup time of a crystal oscillator circuit.
One conventional technique is to initially increase the gain of the amplifier when startup first initiates and the noise voltage levels are smaller. As the differential AC voltage applied at the resonator element increases, the gain of the amplifier is reduced. This conventional technique reduces startup time somewhat, while still relying completely on the internally generated noise as the initial source of resonance. Since the initial level of noise is quite random, the time that it takes for the amplified signal to reach useful levels can vary significantly.
A second conventional approach is to make the capacitors C1 and C2 of FIG. 4 as small as possible for use during startup. Since there is less capacitive load to charge during startup, startup occurs more quickly. After startup, additional capacitive load is connected to the resonating element 401. This conventional method also reduces startup time, but to unpredictable levels since this technique also relies solely on internally generated noise as the initial signal to be amplified.
A third conventional approach is to initiate startup by applying a single pulse across the crystal to start oscillations. This ensures that the crystal will move off its stable state and start oscillating thereby reducing startup time, and provides some improvement in predictability of the startup time.
Nevertheless, some applications are sensitive to variations in startup time. Accordingly, what would be advantageous are crystal oscillator circuits and methods of operating the same in which the startup time is reduced and more predictable. One additional disadvantage of the above-mentioned methods is that they do not compensate for process and temperature variations that are inevitable and that can significantly vary the startup time. Accordingly, what would be further advantageous is if the crystal oscillator circuit provided a quick and predictable startup time that is less dependent on process and temperature variations.
The foregoing problems with the prior state of the art are overcome by the principles of the present invention, which are directed towards an oscillation circuit that is capable of starting up quickly and that has a predictable startup time. The principles of the present invention are also related to methods for starting up the oscillation circuit.
Like conventional oscillation circuits, the oscillation circuit in accordance with the principles of the present invention includes a resonating element that has internally generated noise and that has a resonant frequency. For example, the resonating element may be a crystal, a ceramic or any inductor/capacitor equivalent circuit.
Also like conventional oscillating circuits, the circuit includes an inverting amplifier receiving one terminal of the resonating element as input and having an output terminal connected to the other terminal of the resonating element. A resistor is coupled between the terminals of the resonating element. One capacitor capacitively couples one terminal of the resonating element to a substantially fixed voltage source such as ground, while another capacitor capacitively couples the other terminal of the resonating element to a substantially fixed voltage source such as ground.
Unlike conventional oscillator circuits, an AC current source is configured to generate a differential AC current applied between the two output terminals when active. The differential AC current has a frequency that is within a tolerance of the resonant frequency of the resonant element for a given set of operating conditions. Two buffers connect the differential outputs of the AC current source to respective terminals of the resonating element to thereby shorten startup time. The AC current source may be a temperature compensated ring oscillator to thereby more closely match its frequency to the resonant frequency of the resonating element. A calibration circuit may provide the temperature compensating ring oscillator with calibration to compensate for process variations. This process and temperature compensation allows for an AC differential current having a frequency that is very close to the resonant frequency of the resonating element for various different operating temperatures and process variations. Accordingly, startup time is reduced.
In order to improve the predictability in the startup time, control logic times the activation and deactivation of the AC current source and the buffers so that the differential AC current source is applied to the terminals of the resonating element for sufficient time such that startup would occur even under the worst of a given set of operating conditions or process variations. At the end of that designated time, the AC differential current is disconnected from the terminals of the resonating element thereby allowing the resonating element to generate the reliable AC signal also having the resonating frequency. Accordingly, the predictability of startup times is improved.
Additional features and advantages of the invention will be set forth in the description that follows, and in part will be obvious from the description, or may be learned by the practice of the invention. The features and advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. These and other features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.